In production of semiconductor wafers such as silicon wafers, a polishing process is one of important processes to improve surface roughness and flatness of these wafers. As the precision of devices has recently been increased, there is an increasing need for more precisely flattened semiconductor wafers for use in device fabrication. According to this need, chemical mechanical polishing (CMP) is used as a technique to flatten a surface of semiconductor wafers.
Apparatuses for polishing surfaces of a workpiece such as a silicon wafer may be classified into two types: a single-side polishing apparatus that polishes one of the surfaces of the workpiece at a time and a double-side polishing apparatus that polishes both the surfaces simultaneously.
In a conventional CMP process on a semiconductor wafer (also referred to as a wafer below) with a common single-side polishing apparatus, there is a method to hold an opposed surface to a surface to be polished of the wafer by attaching this opposed surface to a glass plate and so on through an adhesive such as wax.
There is also a method to hold and polish the wafer by using a holding plate provided with a backing pad made of a soft resin sheet foam without using an adhesive such as wax; this method is one of the so-called wax-free polishing or wax-less polishing methods.
In a polishing apparatus 38 shown in FIG. 8, for example, a polishing head 31 including a holding disc body 32 made of ceramic, a backing pad 34 and a template 35 having a circular hole configured to surround a workpiece W that are attached to the holding disc body 32 is used to hold one surface of the workpiece W by bringing the workpiece W into close contact with the backing pad 34 containing water. Then, the polishing head 31 and a turn table 37 are rotated and a polishing agent 33 is supplied to a polishing pad 36 attached to the turn table 37 at the same time as the surface to be polished of the workpiece W is pressed to the polishing pad 36 so as to come into sliding contact with the polishing pad 36. In this way, the surface to be polished of the workpiece W can be finished in a mirror surface.
There is a method of attaching a workpiece to a rigid disc plate with higher flatness through an adhesive, such as wax, as a workpiece holding method to flatten the workpiece by a single-side polishing process. If it is necessary to achieve a uniform polishing stock removal, particularly with respect to the entire surface of the workpiece, the so-called rubber-chuck method is used in which a rubber film is used instead of the rigid disc plate that serves as a workpiece holder, and pressurized fluid such as air is caused to flow to the back of the rubber film so that the rubber film is inflated with a uniform pressure to press the workpiece against the polishing pad (See Patent Document 1).
FIG. 9 schematically shows an exemplary configuration of a conventional polishing head using the rubber-chuck method. This polishing head 102 mainly includes an annular rigid ring 104, a rubber film 103 stuck on the rigid ring 104, and a mid plate 105 combined with the rigid ring 104. A sealed space 106 is defined by the rigid ring 104, the rubber film 103, and the mid plate 105. In addition, an annular template 114 is disposed in the vicinity of a lower surface of the rubber film 103 concentrically with the rigid ring 104. The pressure of the space is adjusted by supplying pressurized fluid with a pressure adjustment mechanism 107 disposed at the center of the mid plate 105. The polishing head also includes a means for pressing the mid plate 105 toward the polishing pad 109, but this means is not shown in the figure.
Patent Document 2 proposes various rubber materials used for the rubber film 103 such as fluororubber, isobutylene-isoprene rubber, chloroprene rubber, polyurethane rubber, and silicon rubber that exhibit physical properties of a hardness of 10 to 100, a tensile strength of 3 to 20 MPa, a tensile elongation of 50 to 1000%, and a thickness of 0.2 to 3 mm.
Patent Document 2 discloses metal materials such as stainless steel and aluminum used for the rigid ring 104.
Patent Document 2 also discloses a method of forming the rubber film 103 on the rigid ring 104 by putting the rigid ring 104 and a flexible rubber lump into a metal mold, heating these to 150° C. to 185° C., and compression molding the resultant under a clamping pressure of 1 to 200 tons.
With the polishing head 102 configured as above, the polishing process is performed in a manner that the workpiece W is held by the lower surface of the rubber film 103 though the backing pad 113, the edge of the workpiece W is held by the template 114, and then the mid plate 105 is pressed to bring the workpiece W into sliding contact with the polishing pad 109 attached to the upper surface of the turn table 108.